1. Field of the Invention
The present invention relates to an optical information-recording medium. In particular, the present invention relates to an optical information-recording medium on which media information including, for example, the name of the manufacturer and the information concerning copyright protection measures is written in a form of prepits.
2. Description of the Related Art
In recent years, DVD (digital versatile disk), which has a recording capacity several times that of CD (compact disk), is widely used as an information-recording medium on which information including, for example, voices and images and so on is recorded. In addition to DVD, those having been already available as commercial products include DVD-R (write-once type digital versatile disk) on which information can be recorded by a user only once, and DVD-RW (rewritable type digital versatile disk) on which information is rewritable. Widespread use of them is considered to become universal as information-recording media having large capacities in future.
Usually, in the case of DVD-R and DVD-RW, the information (hereinafter referred to as “media information”), which includes, for example, the information on the disk manufacturer and the information on the copyright protection measures, is previously recorded on the innermost circumferential portion or the outermost circumferential portion of the disk. The media information is recorded at the final stage of the disk production steps such that the recording layer is modified, for example, by radiating a light beam by using a recording apparatus. On the other hand, a method is disclosed (see, for example, Japanese Patent Application Laid-open No. 2001-67733, pages 5 to 6, FIGS. 1 to 3), in which the media information is not recorded in the recording layer unlike the above, but the media information is previously recorded in a form of emboss pits (hereinafter referred to as “in-groove pits”) on the groove of the substrate at the stage of the production of the substrate of the disk. A part of an optical information-recording medium manufactured by using this method is shown in FIG. 1. FIG. 1A shows a partial magnified plan view illustrating the optical information-recording medium, which schematically depicts an area (hereinafter referred to as “in-groove pit area”) in which in-groove pits are formed.
FIGS. 1B and 1C show a sectional view taken along a line 1B—1B and a sectional view taken along a line 1C—1C shown in FIG. 1A respectively. As shown in FIG. 1B, the optical information-recording medium is formed such that the depth dp″, which ranges to the bottom surface (lowermost surface) 107a of the in-groove pit 107 on the basis of the land surface 101a of the substrate 101 formed with the land and the groove, is deeper than the depth dg″ which ranges to the bottom surface (lowermost surface) 105a of the groove 105 on the basis of the land surface 101a as well. Accordingly, when a recording layer 102 and a reflective layer 103 are formed on the pattern formation surface of the substrate 101, the difference in the surface height of each of layers to be formed appears between the portion at which the in-groove pit 107 is formed and the portion of the groove at which the in-groove pit 107 is not formed. Therefore, it is possible to record the data such as the media information on the groove by utilizing the difference in depth between the in-groove pit portion and the groove portion.
In the case of the substrates for the DVD-R and DVD-RW, pits (hereinafter referred to as “land prepits”) are provided on the land formed between the grooves, on which, for example, the address information of the disk is recorded (see, for example, Japanese Patent Application Laid-open Nos. 09-326138 (pages 1 to 8, FIGS. 3 and 15) and 2001-118288(pages 1 to 8, FIG. 1)). Further, a method is known (see, for example, Japanese Patent Application Laid-open No. 63-153743 (page 278, pages 280 to 282, FIG. 3)), in which the exposure intensity is switched during the process in which the exposure is performed with a pattern corresponding to one pit in the exposure step for a master disk to produce a substrate provided with pits.
In the case of the optical information-recording medium provided with the in-groove pits, as shown in FIG. 1, the in-groove pit is formed such that the width (length in the radial direction of the disk) of those disposed in the vicinity of the middle portion in the track direction (hereinafter abbreviated as “middle portion”) is wider than that of the end in the track direction (i.e., in the groove direction), probably for the following reason. That is, it is considered that the wide width is generated by the increased totalized exposure amount of the laser beam to be radiated at the middle portion of the pattern when the exposure is performed in order to form the pattern corresponding to the in-groove pit on the master disk. Accordingly, the width of the middle portion of the in-groove pit is widened as compared with the width of the groove, and the side wall of the adjoining land is eroded. That is, the width of the land disposed adjacently to the in-groove pit is narrowed. In particular, when the land prepit is formed on the land disposed adjacently to the in-groove pit, it has been hitherto impossible to sufficiently secure the width of the land surface between the in-groove pit and the land prepit. As a result, as shown in FIG. 11B, the following problems have arisen. That is, the jitter of the detection signal of the land prepit is increased, the aperture ratio (AR) does not satisfy the predetermined requested value, and the error rate is increased.
As far as the present inventors know, there has been no instance in which the exposure intensity is controlled for an in-groove pit formation pattern during the exposure for a master disk in order to control the shape of the in-groove pit which is formed to be deeper than the groove.